Diagnostic utilization of positron emission tomography involves administering a radionuclide to a patient, causing millions of positrons to be emitted within the patient. These positrons travel for very short distances, on the order of a few millimeters, and in their travel interact with electrons of similar mass. When positrons and electrons interact, an annihilation event occurs whereby the mass of the positron and electron are annihilated or disintegrated and photons are emitted at substantially 180.degree. with respect to one another. Positrons are positively charged electrons, usually emitted by radionuclides which are unstable because they include an excess of neutrons with respect to a stable state. Positrons lose their kinetic energy in a manner similar to that of electrons. However, when positrons are brought to rest they undergo the phenomenon of annihilation, whereby the positron interacts with an electron, the two particles undergo annihlation, and the masses are converted into energy in form of two photons called the annihilation photons. These two photons travel at about 180.degree. from each other and each carries an energy of appoximately 511 keV. It is through the simultaneous detection of the two annihilation photons that positron-emitting radio-nuclides are of significance in reconstructing a computed tomography image.
The annihilation photons travel the distance required to impinge upon individual detectors positioned in an array about the patient. In the detectors, the energy carried by the annihilation photons is converted to a flash of light, and it is this flash of light which is sensed by photomultipliers located at the ends of the detectors to thereby permit recording of the annihilation which took place in the patient. In the space of a few minutes, hundreds of thousands of such light flashes are generated and electrical signals representing these flashes are processed by a system which may include a specially programmed digital computer so as to form an image of the area under examination.
Annihilation radiation can be uniquely detected by two scintillation detectors connected to a conicidence circuit. In this arrangement, a count is recorded only if both detectors detect the annihilation photons substantially simultaneously. Annihilation events occurring outside a straight line joining the two detectors cannot be recorded, except in a statistically insignificant occurrence, because the annihilation photons are emitted at about 180.degree. from each other. Therefore, the two detectors operated in time coincidence establish a field of view encompassed by the lines joining them.
Numerous positron emission tomography scanners have been described in the prior art. For example, reference is made to U.S. Pat. Nos. 4,057,727 and 4,150,292. The overwhelming majority of prior art positron emission tomography systems incorporate scintillation or imaging detectors, usually of the sodium iodide type, although at least one prior art system employs bismuth germanate crystals. As used herein, the term detector shall describe any detector useful in nuclear medicine imaging techniques. In its simplest form, a positron emission tomography system consists of two detectors facing each other and scanning across the object at distance angles. In order to achieve high efficiency in collecting the radiation, more detectors can be placed around the object. Typically, the design of detector arrays for this purpose is a circle of detectors.
It should be noted that the diagnostic tomographic visualisation of an organ typically requires several tomographic sections. Therefore, tomographs capable of yielding only one section at a time must be provided sequentially with relative movement of the tomograph to achieve each section. This approach is wasteful of radiation, is time consuming, and is often unsuitable for the study of time-dependent dynamic phenomena throughout the organ image. Further, accurate indexing of the apparatus with respect to the patient is difficult. To alleviate this difficulty, prior art positron imaging systems incorporate the ability to provide several sections simultaneously. One system providing simultaneous images is described in the article by Brooks et al. "DESIGN OF A HIGH RESOLUTION POSITRON EMISSION TOMOGRAPH: The Neuro-PET," which appeared in the Journal of Computer Assisted Tomography, volume 4, No. 1, 1980, printed by Raven Press of New York City. The disclosure appearing in that publication is expressly incorporated herein by reference. This Brooks et al system describes an arrangement whereby four circular arrays of bismuth germanate detectors are arranged in four respective planes, each plane being sub-divided into four quadrants. Other PET scanners similarly have four planes of detectors. In all of these prior art systems, however, a problem exists with respect to detecting a cross-slice annihilation event. More particularly, a cross-slice event is defined as a coincidence of gamma ray capture events in two detectors residing in different planes. In the prior art, implementation of cross-slice event data handling is extremely complex and expensive.